Locating a wireless local area network associated with a wireless wide area network

ABSTRACT

An apparatus and method are disclosed in connection with a mobile device locating a wireless local area network (WLAN) associated with a wireless wide area network (WWAN). These concepts involve obtaining and recording maximum and minimum geometric time differences (GTDs) for a pair or pairs of transmitters in the WWAN while coverage is provided by the WLAN, and using these differences to later compare a contemporaneous GTD to determine whether the mobile device is more likely within the coverage area of the WLAN.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application under 35 U.S.C. §120, andclaims the benefit of and priority to application Ser. No. 11/691,416,filed Mar. 26, 2007 and titled “Locating a wireless local area networkassociated with a wireless wide area network”, which claims the benefitof and priority under 35 U.S.C. §119(e) to U.S. Provisional ApplicationNo. 60/792,252, filed Apr. 12, 2006, titled “Support of WLAN access inassociation with WWAN”, both of which are incorporated herein byreference.

BACKGROUND

I. Field of the Invention

The present disclosure relates generally to telecommunications, and moreparticularly, to concepts for locating a wireless local area networkassociated with a wireless wide area network.

II. Background

Wireless networks, such as those supporting GSM, WCDMA, cdma2000 andTDMA, commonly provide wireless coverage over a large geographicarea—for example, a city, metropolitan area, a state or county orsometimes a whole country. Such networks are sometimes referred to asWWANs (Wireless Wide Area Networks). Recently, smaller wireless networksknown as WLANs (Wireless Local Area Networks) have been standardized,for example by the IEEE 802.11 committee, and deployed to cover smallareas with a geographic coverage ranging from a few tens of meters to afew hundred meters. To enable wider coverage using these networks, it iscommon to deploy many such networks in different adjacent areas. In somecases, the ensemble of such multiple WLANs may be considered as a smallWWAN in its own right with each “cell” of the WWAN being supported by asingle WLAN. Such WLANs or WLAN ensembles may be owned and operated byindependent operators or by the same operators who own and run WWANs orby individual subscribers (e.g., home or office WLANs).

For WWAN operators who own WLANs or have a business arrangement withWLAN operators (e.g., including WWAN subscribers who own WLANs), theremay be an advantage in allowing or assisting wireless terminals toaccess such WLANs as a means of extending coverage (e.g., into areasserved by WLANs but not by WWANs) and/or increasing capacity in the casethat existing WWAN resources (e.g., available spectrum) are insufficientto serve all subscribers. For example, extension of coverage may be anadvantage in areas harder to reach by WWANs (e.g., within shoppingmalls, inside homes and offices) while increasing capacity may be anadvantage in the most heavily used areas (e.g., urban environments),during peak usage times and to reduce WWAN resource utilization.

In the event that wireless terminals are allowed or may be required touse certain WLANs when available, it may become necessary to make theterminal aware that a particular WLAN or set of WLANs is or areavailable at any particular location in which the terminal happens tobe. This could be supported by having the wireless terminal performperiodic searches for available WLANs and then, if a suitable WLAN isfound and its signal strength is deemed adequate, have the terminaltransfer its service to the WLAN (e.g., via registration on the WLAN andhandover of any ongoing services, such as a call, that the terminal maybe engaged in). The problem with this simple approach is that anyfrequent scanning for available WLANs may consume excessive resources inthe terminal (e.g., battery) as well as interfere with ongoing services.Furthermore, if maintaining ongoing services has higher priority, it maynot always be possible to adequately scan for available WLANs. It couldthus be an advantage to have other methods of finding available andsuitable WLANs that do not involve excessive resource utilization in aterminal or conflict with the support of existing services.

One such method has been defined to support WLAN access from terminalsusing CDMA, for example cdma2000, WWANs. This method makes use of thefact that transmissions from CDMA base stations can be synchronized.Specifically, the concept makes use of synchronized timing in CDMA basestations which enables a wireless terminal to measure pilot phases fromnearby base stations and use these to indicate its approximategeographic location. By recording pilot phases associated with theavailability of a particular WLAN, the handset can, at a later time,determine the proximity of the same WLAN when it measures the same pilotphases that it had recorded previously. This enables a wireless terminalto perform handover to the WLAN (once it is detected) and help alleviatecongestion on a normal WWAN or extend WWAN coverage.

However, the above concept does not address asynchronous networks likeGSM and WCDMA because information equivalent to CDMA pilot phases is notsupported in these networks in a form that can be directly used.Accordingly, there exists a need for new technology that may be used tohelp locate WLANs in association with both asynchronous and synchronousWWANs.

BRIEF SUMMARY

One aspect of a method for locating a first network is disclosed. Themethod includes obtaining the difference in transmission time of amarker between two transmitters in a second network, measuring thedifference in arrival time of the marker between the two transmitters,and locating the first network based on the difference in transmissiontime and the difference in arrival time of the marker.

An aspect of a wireless terminal configured to operate in a firstnetwork is disclosed. The wireless terminal includes at least oneprocessor configured to obtain the difference in transmission time of amarker between two transmitters in a second network, measure thedifference in arrival time of the marker between the two transmitters,and locate the first network based on the difference in transmissiontime and the difference in arrival time of the marker.

Another aspect of a wireless terminal configured to operate in a firstnetwork is disclosed. The wireless terminal includes means for obtainingthe difference in transmission time of a marker between two transmittersin a second network, means for measuring the difference in arrival timeof the marker between the two transmitters, and means for locating thefirst network based on the difference in transmission time and thedifference in arrival time of the marker.

An aspect of a computer program product is disclosed. The computerprogram product includes computer-readable medium. The computer readablemedium includes code for causing at least one computer to obtain thedifference in transmission time of a marker between two transmitters ina second network, code for causing the at least one computer to measurethe difference in arrival time of the marker between the twotransmitters, and code for causing the at least one computer to locatethe first network based on the difference in transmission time and thedifference in arrival time of the marker.

It is understood that other configurations of the present invention willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein it is shown and described variousconfigurations of the invention by way of illustration. As will berealized, the invention is capable of other and different configurationsand its several details are capable of modification in various otherrespects, all without departing from the spirit and scope of the presentinvention. Accordingly, the drawings and detailed description are to beregarded as illustrative in nature and not as restrictive.

Disclosed is an apparatus and method for a mobile device to locate aWLAN (wireless local area network) using signals from one or more pairsof transmitters. According to some aspects, disclosed is a method for amobile device to locate a wireless local area network (WLAN), the methodcomprising: recording geometric time difference (GTD) statistics for theWLAN comprising: recording a first minimum GTD between a first pair oftransmitters; and recording a first maximum GTD between the first pairof transmitters; computing a first current GTD based on currentmeasurements from the first pair of transmitters; and checking forcoverage within the WLAN based on the first current GTD being betweenthe first minimum GTD and the first maximum GTD.

According to some aspects, disclosed is a mobile device for locating awireless local area network (WLAN), the mobile device comprising aprocessor and memory wherein the memory includes software instructionsto: record geometric time difference (GTD) statistics for the WLANcomprising software instructions to: record a first minimum GTD betweena first pair of transmitters; and record a first maximum GTD between thefirst pair of transmitters; compute a first current GTD based on currentmeasurements from the first pair of transmitters; and check for coveragewithin the WLAN based on the first current GTD being between the firstminimum GTD and the first maximum GTD.

According to some aspects, disclosed is a mobile device for locating awireless local area network (WLAN), the mobile device comprising: meansfor recording geometric time difference (GTD) statistics for the WLANcomprising: means for recording a first minimum GTD between a first pairof transmitters; and means for recording a first maximum GTD between thefirst pair of transmitters; means for computing a first current GTDbased on current measurements from the first pair of transmitters; andmeans for checking for coverage within the WLAN based on the firstcurrent GTD being between the first minimum GTD and the first maximumGTD.

According to some aspects, disclosed is a computer program product,comprising a computer usable medium having a computer-readable programcode embodied therein, said computer-readable program code adapted to beexecuted to implement a method for generating a report, said methodcomprising: recording geometric time difference (GTD) statistics for theWLAN comprising: recording a first minimum GTD between a first pair oftransmitters; and recording a first maximum GTD between the first pairof transmitters; computing a first current GTD based on currentmeasurements from the first pair of transmitters; and checking forcoverage within the WLAN based on the first current GTD being betweenthe first minimum GTD and the first maximum GTD.

According to some aspects, disclosed is a non-volatile computer-readablestorage medium including program code stored thereon, comprising programcode to: record geometric time difference (GTD) statistics for the WLANcomprising program code to: record a first minimum GTD between a firstpair of transmitters; and record a first maximum GTD between the firstpair of transmitters; compute a first current GTD based on currentmeasurements from the first pair of transmitters; and check for coveragewithin the WLAN based on the first current GTD being between the firstminimum GTD and the first maximum GTD.

It is understood that other aspects will become readily apparent tothose skilled in the art from the following detailed description,wherein it is shown and described various aspects by way ofillustration. The drawings and detailed description are to be regardedas illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWING

Various aspects of a wireless communications system are illustrated byway of example, and not by way of limitation, in the accompanyingdrawings, wherein:

FIG. 1 is a conceptual diagram showing a wireless terminal incommunication with a WWAN;

FIG. 2 is a conceptual diagram showing a WLAN connected to a WWANthrough a packet-switched network;

FIG. 3 is a conceptual diagram showing the topology of a WWAN;

FIG. 4 is a conceptual diagram illustrating how the GTD (Geometric TimeDifference) for a pair of BTSs (Base Transceiver Stations) may beobtained;

FIG. 5 is a conceptual diagram illustrating a WLAN coverage area whollyor partially included within an area defined by maximum and minimum GTDvalues for two pairs of BTSs;

FIG. 6 is a conceptual block diagram of a wireless terminal;

FIG. 7 is a flow chart illustrating an example of a procedure forcreating a fingerprint of a WLAN and later acquiring the WLAN;

FIG. 8 is a flow chart illustrating an example of a procedure forlocation a BTS;

FIG. 9 is a flow chart illustrating an example of a procedure forobtaining GTD values for a WLAN;

FIG. 10 is a flow chart illustrating an example of a procedure fordetecting a WLAN associated with a WWAN;

FIG. 11 is a conceptual diagram illustrating a wireless terminalreceiving transmissions from a pair of BTSs;

FIG. 12 is a flow chart illustrating an example of a method for locatinga first network; and

FIG. 13 is a functional block diagram illustrating an example of awireless terminal configured to operate in a first network.

DETAILED DESCRIPTION

In the following description, specific details are given to provide athorough understanding of the embodiments. However, it will beunderstood by one of ordinary skill in the art that the embodiments maybe practiced without these specific detail. For example, circuits may beshown in block diagrams in order not to obscure the embodiments inunnecessary detail. In other instances, well-known circuits, structuresand techniques may be shown in detail in order not to obscure theembodiments.

Also, it is noted that the embodiments may be described as a processwhich is depicted as a flowchart, a flow diagram, a structure diagram,or a block diagram. Although a flowchart may describe the operations asa sequential process, many of the operations can be performed inparallel or concurrently. In addition, the order of the operations maybe re-arranged. A process is terminated when its operations arecompleted. A process may correspond to a method, a function, aprocedure, a subroutine, a subprogram, etc. When a process correspondsto a function, its termination corresponds to a return of the functionto the calling function or the main function.

Moreover, as disclosed herein, a “storage medium” may represent one ormore devices for storing data, including read only memory (ROM), randomaccess memory (RAM), magnetic disk storage mediums, optical storagemediums, flash memory devices and/or other machine readable mediums forstoring information. The term “machine readable medium” includes, but isnot limited to portable or fixed storage mediums, wireless channels andvarious other mediums capable of storing, containing or carryinginstruction(s) and/or data.

Furthermore, embodiments may be implemented by hardware, software,firmware, middleware, microcode, or any combination thereof. Whenimplemented in software, firmware, middleware or microcode, the programcode or code segments to perform the certain tasks may be stored in amachine readable medium such as a storage medium. A processor also mayperform such tasks. A code segment may represent a procedure, afunction, a subprogram, a program, a routine, a subroutine, a module, asoftware package, a class, or any combination of instructions, datastructures, or program statements. A code segment may be coupled toanother code segment or a hardware circuit by passing and/or receivinginformation, data, arguments, parameters, or memory contents.Information, arguments, parameters, data, etc. may be passed, forwarded,or transmitted via any suitable means including memory sharing, messagepassing, token passing, network transmission, etc.

It should also be apparent to those skilled in the art that one or moreelements of a device disclosed below may be rearranged without affectingthe operation of the device. Similarly, one or more elements of a devicedisclosed below may be combined without affecting the operation of thedevice.

1. Wireless Network Environment

FIG. 1 is a conceptual diagram showing a wireless terminal 102 incommunication with a WWAN 100. The wireless terminal 102 may be anysuitable wireless terminal, including by way of example, a mobile orcellular telephone, a personal digital assistant (PDA), a portabletelevision, a personal computer, a laptop computer, a digital camera, adigital camcorder, a game console, a portable audio device, a portableradio, transceiver, modem, or any other suitable device capable ofaccessing the WWAN 100. The wireless terminal 102 may be referred to bythose skilled in the art as a handset, wireless communications device,user terminal, user equipment, mobile station, mobile unit, subscriberunit, subscriber station, mobile radio, radio telephone, wirelessstation, wireless device, or some other terminology. The variousconcepts described throughout this disclosure are intended to apply allsuitable wireless terminals regardless of their configuration andspecific nomenclature.

A typical WWAN 100 is a cellular network. A cellular network is anetwork in which the geographic coverage region is broken up into cells.Within each cell is a BTS (Base Transceiver Station) that communicateswith wireless terminals. For clarity of presentation, a single BTS 104is shown with a backhaul connected to a BSC (Base Station Controller)106, however, in real world applications, the BSC 106 will supportbackhaul connections with many BTSs. The BSC 106 is used to manage andcoordinate the BTSs in the WWAN 100 so multiple wireless terminals cancommunicate. The WWAN 100 may also be connected to additional networksthrough one or more suitable gateways. In the example shown in FIG. 1,the WWAN 100 is connected through a MSC (Mobile Switching Center) 108 toa circuit-switched network 110, such as a PSTN (Public SwitchedTelephone Network). The WWAN 100 may also be connected to a packet-basednetwork 112, such as the public Internet, through a SGSN (ServingGeneral Packet Radio Service Support Node) 113 and a GGSN (GatewayGeneral Packet Radio Service Support Node) 115. The SGSN 113 facilitatesexchanges of packets between the BSC 106 and the GGSN 115 and alsoperforms mobility management for the wireless terminals 102. The GGSN115 performs routing functions and exchanges packets with thepacket-based network 112.

FIG. 2 is a conceptual diagram showing a WLAN 114 connected to the WWAN100 through the packet-based network 112. The WLAN 114 may provideextended coverage to wireless terminals and/or increase the capacity ofthe WWAN 100 using any suitable wireless protocol including IEEE 802.11,Ultra Wide Band (UWB), or Bluetooth, just to name a few. The actualwireless protocol implemented in any particular WLAN will depend on thespecific application and the overall design constraints imposed on theoverall system. A MGW 116 (Media Gateway) is used to help interface theWLAN 114 to the circuit-switched network 110—e.g. to support voicecalls.

FIG. 3 is a conceptual diagram showing the topology of a WWAN 100. Inthis example, the WWAN 100 is broken up into five cells 202 a-202 e. ABTS 104 a-104 e is located in each cell 202 a-202 e, respectively. Inaddition, three WLANs 114 are dispersed throughout the geographiccoverage region of the WWAN 100. As the wireless terminal 102 movesthroughout the WWAN 100, it may employ various techniques to learn ofthe existence of WLANs in different locations. These techniques make useof transmissions from a pair of BTSs belonging to the WWAN 100 ormultiple WWANs. These techniques may be applied to both asynchronous andsynchronous WWANs.

The techniques make use of the concepts of OTD (Observed TimeDifference), RTD (Real Time Difference) and GTD (Geometric TimeDifference). The OTD is the difference in the arrival times at awireless terminal of specifically tagged or otherwise identified oridentifiable signals from a pair of BTSs. For example, for GSM, the OTDcould be the time following detection at a wireless terminal of thestart of one GSM frame from one BTS until the detection at the wirelessterminal of start of the next GSM frame from another BTS. The OTD isclosely associated with the RTD, which provides the OTD without theeffect of propagation delay, for example, the time following the startof any GSM frame at one BTS until the start of the next GSM frame at theother BTS. OTD equals RTD for any wireless terminal that is equidistantfrom the pair of BTSs.

An example will now be given with reference to FIG. 4. The differencebetween the OTD observed by the wireless terminal 102 at any location Lfor a pair of BTSs P and Q and the RTD between these BTSs provides theGTD. The relationships can be represented as follows.

Let: M_(R)=a transmission marker (e.g., start of a frame in GSM) fromBTS R

-   -   T(M_(R))=Absolute transmission time of marker M_(R) at BTS R    -   A(M_(R))=Absolute arrival time of marker M_(R) at Location L    -   where R=P or Q

then: OTD=A(M _(R))−A(M _(Q))

RTD=T(M _(R))−T(M _(Q)).

Let: GTD=OTD−RTD   (1-1)

then: GTD=(A(M _(p))−A(M _(Q)))−(T(M _(p))−T(M _(Q)))

GTD=(A(M _(p))−T(M _(p)))−(A(M _(Q))−T(M _(Q)))

$\begin{matrix}{{GTD} = \frac{\left( {D_{P} - D_{Q}} \right)}{c}} & \left( {1\text{-}2} \right)\end{matrix}$

where: D_(R)=distance between location L and BTS R; and

-   -   c=signal propagation speed (e.g., speed of light).

Equation (1-2) shows that the GTD equals the difference between thedistances from location L to each BTS divided by c and hence depends onthe geometry of the locations of the BTSs and the location L. Otherlocations with the same GTD as L all lie along a certain hyperbola thatpasses through L, with the property that the difference in the distancesto the two BTSs from any point is fixed according to Equation (1-2).

For modern wireless networks, the transmission timing of any BTS isnormally very stable and accurate, changing by small amounts (e.g., afraction of 1 GSM bit in the case of GSM) over a period of a day ormore. This means that the RTD between any pair of BTSs will normally bealmost constant over a similar interval which in turn means that if theRTD can be determined in some way at intervals of a few hours or a day,then just by making OTD measurements, a wireless terminal will be ableto derive GTD values for the various locations it occupies within rangeof the pair of BTSs. OTD measurements are typically not difficult tomake or necessarily very resource intensive because a wireless terminalmay need to monitor transmissions from nearby BTSs in order to supportnormal wireless operation including handover, cell change and, in thecase of the serving BTS, radio contact. The additional resource use tomeasure signals that are already being monitored would then be small.Because GTD values depend on geometry, it would then be possible for anywireless terminal to use a set of GTD values obtained for multiple pairsof BTSs as a means of characterizing any location that it occupied. Anytwo GTD values obtained for a particular location from two differentpairs of BTSs are normally sufficient to derive the location exactlygiven the locations of each of the BTSs.

In order to characterize the geographic area in which a particular WLANhas coverage, a wireless terminal could create a fingerprint containinga set of GTD values for pairs of BTSs whose coverage overlaps part orall of the WLAN coverage area. From the obtained GTD values, thewireless terminal could derive statistics, for example, the minimum andmaximum GTD values for each pair of BTSs that applied to the entire WLANcoverage area that the wireless terminal had visited. This isillustrated in FIG. 5 for two pairs of BTSs.

Because BTS coverage in a WWAN is typically much more extensive than thecoverage area of a WLAN and because each BTS can usually be detected bya wireless terminal well outside its normal service area, it will oftenbe the case that GTD values for several pairs of BTSs will be obtainablethroughout the entire coverage area of the WLAN. Even if that is not thecase, it may still be possible to obtain GTDs for a number of pairs ofBTSs at any location in the WLAN coverage area. By recording the minimumand maximum GTDs thus obtained for each pair of BTSs or other statisticsfor the GTDs, it may be possible on a later occasion when the wirelessterminal returns again to the same WLAN coverage area to use the storedGTD statistics as a means of knowing that wireless terminal may bewithin the WLAN coverage area without having to scan for the WLAN on acontinuous basis. This would then enable efficient acquisition of theWLAN.

2. Wireless Terminal

FIG. 6 is a conceptual block diagram illustrating an example of awireless terminal. The wireless terminal 102 may be configured to createa fingerprint for a particular WLAN by recording GTD statistics for thatWLAN and later make use of them to efficiently acquire the WLAN. Thewireless terminal 102 may be implemented in a variety of fashions. Inthe example shown in FIG. 6, the wireless terminal 102 includes aprocessor 602 which communicates with a number of peripheral devices viaa system bus 604. Although the processor 602 is shown as a single entityfor the purposes of explaining the operation of the wireless device 102,those skilled in the art will understand that the functionality of theprocessor 602 may be implemented with one or more physical processorsand storage media. By way of example, the processor 602 may beimplemented with a microprocessor, or an embedded processor, whichsupports software. Alternatively, the processor 602 may be implementedwith application specific integrated circuits (ASIC), field programmablegate arrays (FPGAs), programmable logic components, discrete gates ortransistor logic, discrete hardware components, or any combinationthereof, either alone or in combination with a microprocessor and/or anembedded processor. Accordingly, the term “processor” is to be construedbroadly to cover one or more entities in a wireless terminal that iscapable of performing the various functions described throughout thisdisclosure. The manner in which the processor 602 is implemented willdepend on the particular application and the design constraints imposedon the overall system. Those skilled in the art will recognize theinterchangeability of hardware, software, firmware, middleware,microcode, and similar configurations under the circumstances, and howbest to implement the described functionality for each particularapplication.

The wireless terminal 102 also is shown with a WWAN transceiver 606 anda WLAN transceiver 608. The WWAN transceiver 606 may be configured tosupport any suitable wireless protocol including, by way of example,GSM, WCDMA, cdma2000, and TDMA. The WLAN transceiver 608 may also beconfigured for different wireless protocols including 802.11, UWB,Bluetooth, and others. In the configuration shown in FIG. 6, eachtransceiver 608, 608 is shown as a separate entity, but may beintegrated into a single entity or distributed across other entities inalternative configurations of the wireless terminal 102.

3. Procedure to Record GTDs and Later Acquire WLAN

FIG. 7 is a flow chart illustrating an example of a procedure forcreating a fingerprint of a WLAN and later acquiring the WLAN. Thisprocedure may be performed by a processor as described in connectionwith FIG. 6, or some other entity (or entities) within the wirelessterminal. In step 702, the locations of BTSs are obtained. This step isconditional on the procedure used in step 704 to obtain RTD values andmay not always be needed. In step 704, the RTDs between pairs of BTSsare obtained either making use of the BTS locations obtained in step 702or without them. Using the RTD values obtained in step 704, GTD valuesand statistics (e.g., minimum, maximum and mean GTD values) are obtainedand recorded, in step 706, for a set of BTS pairs providing coveragewithin the coverage area of some WLAN. At some later point in time, theRTDs between pairs of BTSs are again obtained by repeating step 704.Repeating step 704 allows for change in RTD values over any significanttime period. After step 704 is repeated, GTD values are again obtained,in step 708, for the pairs of BTS used in step 706 and are compared tothe GTD statistics recorded in step 706. For example, it may be verifiedthat the GTD values obtained in step 708 lie in between the minimum andmaximum GTD values obtained in step 706 for the corresponding pairs ofBTSs. When the obtained GTD values match the recorded GTD statistics,the proximity of the associated WLAN may be assumed and an attempt maybe made to acquire it.

3.1 Obtaining BTS Locations

FIG. 8 is illustrating an example of a procedure for obtaining the BTSlocations in step 702 of FIG. 7. In this example, a wireless terminalobtains the locations of a set of nearby BTSs. The procedure may berepeated to obtain the locations of different sets of BTSs when thewireless terminal is at other locations. The minimum number of BTSs inthe set is two, but more BTSs can be included to enable the locations ofmore BTSs to be obtained in a single execution cycle for the procedure.

In step 802, the wireless terminal may measure OTD values between pairsof nearby BTSs belonging either to the WWAN it is currently accessing orto some other WWAN providing coverage in the same location. The OTDsmeasured by the wireless terminal will depend on the wireless terminal'slocation and may contain random measurement errors that could be quitesmall. Any particular set of measured OTDs will be related to thelocation of the wireless terminal at the time the measurements are made.The relationship of the OTDs to the location of the wireless terminal isdescribed in more detail in section 5 of this disclosure.

In step 804, the wireless terminal decides if the OTD measurements fromstep 802 have been made at a new location. This will always be deemed tobe true when OTDs are first measured for any new set of BTSs (i.e.,whenever this step is first executed for a particular set of BTSs). Whenthis step is executed subsequently, it may be deemed to be true if theOTD measurements differ by some minimum amount from some or all previousOTD measurements that the wireless terminal has recently stored for thesame set of BTSs (e.g., stored within the last day or last few hours).For example, a significant OTD change may be assumed if the change issignificantly greater than the expected error in measuring the OTD,since in that case, the change can be determined accurately. If OTDmeasurements from step 802 do not differ significantly from previousmeasurements, the measurements are not stored and the wireless terminalmay wait for some interval and then repeat the step 802. If themeasurements do differ, they are stored and the wireless terminal nextobtains its own location in step 806 using, for example, a satellitenavigation system such GPS or Galileo and possibly with assistance fromthe serving network, as in A-GPS. Preferably, the OTD measurements instep 802 and the location in step 806 are obtained at nearly the samepoint in time so that the OTD measurements in step 802 can be assumed tohave been made at the location determined in step 806. The wirelessterminal location is then stored in association with the OTDmeasurements. In the following step 808, the wireless terminal decidesif it has OTD measurements from step 802 for a sufficient number oflocations from step 806. This will be generally true if the wirelessterminal has OTD measurements for at least one pair of BTSs at five ormore different wireless terminal locations as shown in section 5 of thisdisclosure. If it is deemed to be true, then in step 810, using the OTDmeasurements made at different known wireless terminal locations, thewireless terminal may determine the BTS locations as shown in section 5of this disclosure. If the wireless terminal does not yet havemeasurements for enough locations, then it returns to step 802, whereina new set of OTD measurements may be later obtained.

In the event that the wireless terminal 102 is unable to measure OTDvalues between a pair of BTSs in five or more different location, analternative procedure may be used to obtain the BTS locations. With thisprocedure, the wireless terminal would measure the association betweenthe received transmission timing of some BTS and some universal timelike GPS time, or its own internal clock time. If this is done by thewireless terminal at three separate locations for which the wirelessterminal also obtains its location coordinates (e.g., using GPS orA-GPS), then it also becomes possible to obtain the location (e.g.,latitude/longitude coordinates) of the BTS. To show how this can work,suppose that the wireless terminal associates a received transmissiontime T1 from the BTS with universal time Ul at location L₁ andsubsequently associates a received transmission time T2 from the BTSwith universal time U2 at location L₂. Suppose further that BTStransmission times T1 and T2 and universal times U1 and U2 are convertedinto the same time units—for example, into GSM bit times in the case ofGSM. The wireless terminal may now calculate the received transmissiontime (T2−U2+U1) that would have been observed at location L₂ atuniversal time U1. This transmission time may have to be adjusted forany wraparound effects if transmission timing for the BTS is definedcyclically (as is normally the case for a WWAN). The receivedtransmission time T1 at location L₁ and received transmission time(T2−U2+U1) at location L₂ both apply at the same universal time U1.Hence their difference, (T2−T1−U2+U1), must equal the difference in thepropagation times from the BTS to each location. This time differencecan be converted to a distance difference by multiplying by the signalspeed. This leads to the following equation:

$\begin{matrix}{\left( {{T\; 2} - {T\; 1} - {U\; 2} + {U\; 1}} \right) = \frac{\sqrt{\left( {y_{2} - y_{B}} \right)^{2} + \left( {x_{2} - x_{B}} \right)^{2}} - \sqrt{\left( {y_{1} - y_{B}} \right)^{2} + \left( {x_{1} - x_{B}} \right)^{2}}}{c}} & \left( {3\text{-}1} \right)\end{matrix}$

where: (x_(B), y_(B)) are horizontal x, y coordinates of the BTS; and

-   -   (x_(i), y_(i)) are horizontal x, y coordinates of location L_(i)        (where i=1, 2).

Equation (3-1) locates the BTS on a hyperbola defined by the x,ycoordinates of the two locations L₁ and L₂ and the time difference(T2−T1−U2+U1). If the wireless terminal then obtains the associationbetween universal timing U3 and the received BTS transmission timing T3at a third location L₃ with coordinates (x₃, y₃), it will be possible toobtain a second equation like Equation (3-1) in which U3, T3, x₃, y₃replace U2, T2, x₂, y₂, respectively. This will be enough to solve forthe coordinates of the BTS. This procedure has the advantage that thelocation of any BTS can be obtained using measurements from threelocations without necessarily being able to receive signals from anyother BTS. The procedure is also valid when the universal times recordedby the wireless terminal are replaced by internal clock times.

In an alternative procedure to obtain the location of a BTS, a wirelessterminal could obtain the signal propagation times P1 and P2 between aparticular BTS and the wireless terminal at two different locations L₁and L₂. In some wireless technologies (e.g., GSM), the network (e.g.,BTS) may be required to provide the wireless terminal with a value(e.g., derived by the BTS) equal to or related to the signal propagationtime between the BTS and wireless terminal in order for the wirelesstechnology to operate correctly. In GSM, the value provided by the BTSis for the timing advance (TA) which equals the round trip propagationdelay or twice the one way propagation delay. This TA value is providedperiodically to the wireless terminal whenever the wireless terminal isreceiving service from the network (e.g., during a call, whenregistering with the network or performing a periodic registrationupdate). Whenever a new TA value is provided (or whenever a new TA valueis provided that differs significantly from previous TA values), thewireless terminal may also determine its location coordinates (e.g.,using GPS or A-GPS). If the wireless terminal does this at two differentlocations for which the TA has been provided, it can determine valuesfor the propagation times P1 and P2 at known locations L₁ and L₂. Thewireless terminal may then calculate the distances to the BTS from thelocations L₁ and L₂ as P1*c and P2*c where c is the signal propagationspeed (e.g., radio transmission speed in air). Given the locationcoordinates for L₁ and L₂, this enables the location of the BTS to bedetermined This procedure has the advantage that measurements for asingle BTS are needed at only two locations.

3.2 Obtaining RTD Values for Pairs of BTSs

Once the BTS locations are determined, the wireless terminal obtains RTDvalues between selected pairs of BTSs (see step 704 in FIG. 7). Theselection of which pairs of BTSs to use will be described later insections 3.3 and 3.4 of this disclosure.

In order to obtain RTDs, the wireless terminal may obtain its ownlocation, for example using GPS, A-GPS or Galileo, and at the same time,or almost the same time, measures the OTDs between one or more pairs ofBTSs. Using its own location and the locations of each BTS, the wirelessterminal can obtain the distance to each BTS and hence the GTD for anypair of BTSs using Equation (1-2). Equation (1-1) can then be used toobtain the RTD for any pair of BTSs from the GTD and measured OTD.

In an alternative procedure, RTD values (and possibly BTS coordinates)might be provided to a wireless terminal by the network (e.g., via abroadcast service or in system information messages) thereby obviatingthe need for the wireless terminal to calculate these values using theprocedures described herein. This will also obviate the need for thewireless terminal to be able to obtain its own location.

A wireless network supporting, for example GSM or WCDMA, can obtain RTDvalues using separate LMUs (Location Measurement Units) belonging to thenetwork. An LMU is a device positioned at a fixed known location whichmeasure OTDs between pairs of BTSs and calculates RTDs using informationabout its own location and the locations of the BTSs. RTDs can also beobtained if each BTS contains a GPS receiver and is able to synchronizeor associate its transmission timing with GPS timing. Some GSM networksare in the process of being upgraded with GPS receivers in BTSs toenable reduced interference and higher capacity. This capability couldthus be extended to deriving RTD values. Where LMUs or GPS receivers arenot supported, the network could utilize GPS position estimates and OTDmeasurements provided by individual wireless terminals (in order tocalculate RTD values given that BTS coordinates can be already known).The network will have an advantage in being able to access suchmeasurements from many wireless terminals in each cell on a continuousbasis which means RTD values can be kept constantly updated and providedto other wireless terminals as and when needed or requested.

In some cases, the BTSs in a wireless network may be kept accuratelysynchronized in order to provide or improve service (e.g., reduce theinterference level for wireless terminals caused by non-serving BTSs).The methods used to achieve synchronization can include using a GPSreceiver at each BTS to align the BTS transmission timing with universalGPS time. Other methods may also be used. If a wireless terminal isaware that BTSs are synchronized (e.g., because this is essential forthe network technology to operate correctly or because this informationhas been provided by the network or by the network operator) then itneed not perform any measurements to obtain RTD values and can insteadassume that all RTD values are zero.

A third procedure may be employed to obtain RTD values if a networkcannot provide RTD values explicitly or implicitly and if it ispreferred to avoid using a procedure which may require having tocalculate BTS locations. In this procedure, a wireless terminal couldmake use of the signal propagation delay P1 between itself and a BTS ifthis is provided directly or indirectly by the network, as for examplein the case of GSM where a TA value is provided. If the wirelessterminal can then accurately associate the received transmission timingT1 of the BTS with some universal time U1, such as GPS time, (or itsinternal clock time) then it will be able to derive the associationbetween transmission timing at the BTS with universal time (or internalclock time) which, in this example, will be a transmission time (T1+P1)at a universal time U1 if T1 and P1 are expressed in the same timeunits. If the wireless terminal later repeats this procedure for anotherBTS, and determines a propagation delay P2, transmission time T2 anduniversal time U2, if can determine the RTD between the BTSs as thedifference between their transmission timings adjusted to a commonuniversal time. In this particular example, the RTD would be(T2+P2−U2)−(T1+P1−U1) if all values are first converted to use a commontime unit.

3.3 Recording GTD Values for Each WLAN

FIG. 9 is illustrating an example of a procedure to obtain the GTDvalues for each WLAN (see step 706 in FIG. 7). This procedure may beperformed once for any particular WLAN or may be repeated on multipleoccasions to increase the amount of information collected for the WLAN.This procedure may be instigated in WLAN areas of interest. For example,it may be instigated in areas within which a wireless terminal spendssignificant time like the home area, office area or a frequently usedshopping mall. Such areas can be deduced from usage of the same WWANserving cell for long periods, from usage of the same serving cell onmany different occasions, from proximity of the same WWAN cell for longperiods (e.g., if a cell is always included in the set of cells forwhich handover related measurements are ordered from the serving cell)or on many different occasions or from some combination of theseconditions. This procedure could also be instigated due to other factorssuch as when a particular WLAN is observed or is used over a long periodor on multiple occasions or when ordered by the wireless terminal useror by the serving or home network.

In step 902, the wireless terminal determines that it is within coverageof a particular WLAN access point of interest (e.g., belonging to orassociated with its home network operator). The wireless terminal alsodetermines that it is within the coverage area of a number of cell sitesthat will be each associated with their own BTS. The wireless terminalcan determine the identity of each cell site and the associated BTS bysearching for a broadcast signal from each BTS above a certain minimumsignal strength or from information provided by its current serving BTS.For example, the BTSs could be those indicated by a GSM or WCDMA servingBTS whose signal strength and quality the wireless terminal may berequired to measure in order to support handover. For WCDMA, the BTSs(also called Node Bs) could include those currently supporting a softhandover for the wireless terminal. The BTSs and their associated cellsites could all belong to the same network or could belong to differentnetworks. The BTSs could also belong to different networks supportingdifferent wireless technologies (e.g., GSM and WCDMA). Normally, theserving cell and associated serving BTS would belong to this set.

In the next step 904, the wireless terminal may obtain the location ofeach of the BTSs determined in step 902, for example using one of themethods discussed in section 3.1 above. This may have occurred at someprevious time or could occur as a prelude to the rest of this procedure.This step is not needed if BTS locations are not needed to obtain RTDvalues in step 906.

In the next step 906, the wireless terminal may obtain RTD valuesbetween pairs of the BTSs determined in step 902 using any of theprocedures described in section 3.2 above. The pairing of BTSs can bedetermined in different ways. For example, if all BTSs support the samewireless technology (e.g., GSM or WCDMA), one BTS could be common toevery pair—for example the serving BTS or the BTS with the strongestsignal. If the BTSs support different wireless technologies, a singleBTS could be chosen for each technology and paired with each of theother BTSs supporting the same technology.

In the next step 908, the wireless terminal measures OTDs between thepairs of BTSs and in step 910 obtains GTDs from the OTDs and the RTDs asdefined in Equation (1-1) (and as described in greater detail in section5 of this disclosure). If steps 908 and 910 are executed immediatelyfollowing step 906, the GTDs may be obtained directly from the locationof the wireless terminals and the locations of the BTSs used in step906, as defined in Equation (1-2) and as further described in section 5of this disclosure.

In step 912, the wireless terminal pauses (e.g., runs a timer) and then,in step 914, verifies if it is still within coverage of the WLANidentified in step 902. If not, the procedure terminates with thewireless terminal retaining the GTD values that it has obtained earlierin step 910. Otherwise, the wireless terminal repeats the OTDmeasurements of step 908 provided it is still within coverage of theWLAN identified in step 902. The wireless terminal then obtains a newset of GTDs by repeating step 910.

The result of this procedure is a fingerprint of a particular WLANcontaining a set of GTD values for particular pairs of BTSs whosesignals were received within the coverage area of that WLAN. For eachpair of BTSs, the mean GTD value may then be obtained from the maximumand minimum GTD values. The minimum and maximum GTD values define ageographic area between two hyperbolae, one hyperbola associated withthe minimum GTD value and the other associated with maximum GTD value,that would contain the locations of the wireless terminal when itobtained the OTD values in step 908 from which the GTD values werederived. This is illustrated in FIG. 5. Because each of the wirelessterminal locations is associated with reception of the particular WLAN,the area also contains some or all of the coverage area of theparticular WLAN. Some of the coverage area of the WLAN may be missing ifthe wireless terminal did not visit the entire coverage area. The sameapplies for the minimum and maximum GTDs obtained by the MS for otherpairs of BTSs.

In order to ensure that the complete coverage area of the WLAN isincluded within the area defined by the maximum and minimum GTD values,the wireless terminal could simply continue to add more GTD values foreach BTS pair for a particular WLAN by repeating this procedure at latertimes when it is within the same WLAN coverage area. This providesfurther opportunity for the wireless terminal to visit more of the WLANcoverage area. However, this may not be very efficient and the wirelessterminal may not visit the entire coverage area. Another method ofensuring the entire coverage is included would be to calculate theexpected difference D between the minimum and maximum GTD values thatwould be obtained for the entire coverage area assuming that the extentof this coverage area can be known from knowledge of the particular WLAN(e.g., the WLAN technology). This can be done, for example, using theequations in section 5 of this disclosure. The minimum and maximum GTDscan then be increased so that the difference between them is D orslightly more than D to allow for measurement errors. For example, themaximum GTD could be set to the mean GTD plus D/2 and the minimum GTDcould be set to the mean GTD minus D/2. Another means of ensuring theWLAN coverage is included would just be to add and subtract some otherfixed value V from the mean GTD to obtain modified maximum and minimumGTD values. To allow for the likelihood that the mean GTD does notrepresent the center of the WLAN coverage area, V could exceed D/2,although need not exceed D. If the difference between the minimum andmaximum measured GTDs is d, setting V to (D−d/2) would normally ensurethat the entire WLAN coverage area is included within the resultingmodified maximum and minimum GTD values.

The wireless terminal then stores in association with the WLAN the setof BTSs determined in step 902, their associated cell identities, theminimum and maximum GTDs obtained between pairs of BTSs and possiblyother GTD statistics.

3.4 WLAN Detection

FIG. 10 is a flow chart illustrating an example of a procedure used forWLAN detection in step 708 of FIG. 7. With this procedure, the wirelessterminal makes use of the minimum and maximum GTD values and possiblyother GTD statistics previously obtained using the procedure describedabove in section 3.3 for a particular set of BTSs (each associated witha particular cell site) for one or more WWANs as a means to detect thatit may be within the coverage area of a certain WLAN.

This procedure is instigated when the wireless terminal detects two ormore BTSs, for which GTD values have been obtained for some WLAN ofinterest. For example, the wireless terminal may determine that thecurrent serving BTS is one of these BTSs. Preferably, the wirelessterminal also verifies that signals can be received from many or allBTSs for which GTD values were previously obtained within the WLANcoverage area as described above in section 3.3.

In a first step 1002, the wireless terminal instigates any of theprocedures in section 3.2 to obtain the RTD values betweens the samepairs of BTSs for which GTD values were stored to create a fingerprintas described earlier in section 3.3.

In the next step 1004, the wireless terminal measures OTD values betweenthe pairs of BTSs. Then in step 1006, the wireless terminal obtains theGTD values between the pairs of BTSs from the OTD and RTD values asdefined in Equation (1-1). In step 1008, the wireless terminal comparesthe GTD values obtained in step 1006 with the minimum and maximum GTDvalues (or other GTD statistics) previously obtained as describedearlier in section 3.3 for each BTS pair. If all (or most of) theobtained GTD values lie in between the corresponding minimum and maximumGTD values, the wireless terminal may conclude in step 1010 that it maybe within the coverage area of the WLAN and may then attempt to acquiresignals from the WLAN in step 1012. If the wireless terminal is able toreceive a signal from the WLAN, it may then decide in step 1014 toattempt to handover any ongoing services (e.g., call) to the WLAN instep 1016 or simply camp on the WLAN signal if currently idle. If in,step 1010, the GTD values do not all lie within the correspondingminimum and maximum GTD values or if, in step 1012, the wirelessterminal is unable to acquire a signal from the WLAN, the wirelessterminal may repeat step 1004 a short while later. In this case, as thewireless terminal moves around, it may eventually enter the coveragearea of the WLAN. When that occurs, the GTD values obtained in step 1006would generally all lie within the earlier obtained minimum and maximumGTD values, allowing the wireless terminal to know that it may be withincoverage and to attempt to acquire the WLAN. If the wireless terminalinstead moves away from the coverage area of the WLAN, it may eventuallynot be able to detect, or not be able to measure OTD values between,some or all of the BTSs for which GTD values have been obtainedpreviously for the WLAN. If this occurs, the wireless terminal may exitthe procedure (e.g., after attempting unsuccessfully to re-execute step1004).

4. Extension to Multiple WLANs

The various concepts and techniques described herein enables a wirelessterminal to determine when it is likely to be within the coverage areaof a specific WLAN. These concepts and techniques can be extended toenable the wireless terminal to determine when it is likely to be withinthe coverage area of any one of a number of WLANs by repeating theprocedures described above either separately for each WLAN or inparallel. The wireless terminal can also be enabled to determine that,at any location, it is likely to be within the coverage area of morethan one WLAN. In this case, the procedure for detecting a WLANdescribed in section 3.4 above would be repeated, at the same location,for more than one WLAN (either serially or in parallel) and could yieldthe result that the wireless terminal is within the coverage of two ormore WLANs. The wireless terminal would then have the option ofattempting to select any of these WLANs (e.g., the most preferred WLAN).

5. Extension Using WWAN Assistance

The fingerprint information for any WLAN that may be created by awireless terminal using, for example, the method described in section3.3, may also be provided either in part or completely to the wirelessterminal by the network—for example by the home WWAN for the wirelessterminal or by the serving WWAN. This fingerprint information mayinclude the identities of one or more pairs of BTSs that can be detectedwithin the WLAN coverage area and GTD statistics for each pair of BTSs(e.g., mean, minimum and maximum GTDs) that apply to the WLAN coveragearea. There may be a benefit to providing this information to avoid thetime and resource usage in each wireless terminal that would otherwisebe needed to obtain this information. In addition, the information canbe more reliable. For example, a network operator may know the precisecoordinates of its own BTSs and possibly of BTSs belonging to othernetworks and may know the locations of WLANs (e.g., the locationcoordinates of WLAN Access Points). The network operator may then beable to precisely calculate GTD statistics without needing to performany measurements. The calculated information may then be provided towireless terminals on request or as decided by the network or as part ofsystem or other broadcast information.

In some cases, a network operator may not know the locations of someWLANs or the coordinates of some BTSs. In that case, the networkoperator might not be able to calculate any fingerprint information(e.g., GTD statistics) for some WLANs and/or might not be able tocalculate some fingerprint information for other WLANs—for example,fingerprint information comprising information (e.g., GTD statistics)related to certain pairs of BTSs. In other cases, a network operator maynot wish to support the overhead of discovering WLAN and BTS locationsor the overhead of performing fingerprint calculations. In these cases,wireless terminals may obtain fingerprint information for WLANs using,for example, the method of section 3.3. Wireless terminals may thenprovide a network, e.g. the home network or current serving network,with the fingerprint information that was obtained. The network may thencombine the fingerprint information received from many wirelessterminals to obtain fingerprint information for a greater number ofWLANs—for example, for all WLANs visited by the reporting wirelessterminals. In addition, the network may combine fingerprint informationreceived from different wireless terminals concerning the same WLAN inorder to obtain more reliable fingerprint information for this WLAN. Forexample, if two different wireless terminals provide GTD statistics forthe same WLAN related to different pairs of BTSs, the network cancombine the information and obtain GTD statistics for the combined setof BTS pairs. Also, if both wireless terminals provide GTD informationfor the same WLAN concerning the same pairs of BTSs, the network maycombine the information for each pair of BTSs. For example, the networkmay take the higher of two provided maximum GTD values, the lower of twoprovided minimum GTD values and/or the mean of two provided mean GTDvalues. If many wireless terminals provide GTD information for the sameWLAN concerning the same pairs of BTSs, the network may also detecterroneous GTD information by looking for GTD information from anywireless terminal that is different to or not consistent with the GTDinformation provided by the other, or most of the other, wirelessterminals.

The combined fingerprint information may then be provided to individualwireless terminals—e.g. on request, as decided by the network or throughbroadcast. With this extension, a wireless terminal will be able toacquire fingerprint information much faster than it can alone and for amuch larger number of WLANs. Furthermore, the acquired fingerprintinformation can be more accurate and reliable.

6. Obtaining BTS Locations and RTD and GTD Values Using OTD Measurements

An example of a procedure for locating both wireless terminals and BTSsusing OTD measurements will now be described. The procedure can beextended to enable derivation of RTD values and GTD values also.

If a wireless terminal measures the OTD between pairs of nearby BTSsbelonging to some WWAN—and provided the WWAN technology supportstransmission with implicit or explicit timing information such as theexplicit frame and bit numbering used in GSM—the location of theterminal can be obtained by trilateration. This method is employed bythe Enhanced Observed Time Difference method (E-OTD) for GSM wirelessnetworks, the Observed Time Difference of Arrival method (OTDOA) forWCDMA wireless networks and the Advanced Forward Link Trilaterationmethod (A-FLT) for cdma2000 and IS-95 wireless networks. With the E-OTD,OTDOA and A-FLT methods, the locations of BTSs are assumed to beavailable.

The procedure makes use of measurements by any wireless terminal of OTDsbetween pairs of nearby BTSs, preferably at the same time or almost thesame time. The procedure may require that the OTD capable terminal canalso be located using a separate independent, accurate and reliablemethod such as GPS or A-GPS.

To exemplify the procedure, suppose that BTSs P and Q, for example theBTSs P and Q shown in FIG. 4, each transmit a sequence of information(e.g., binary encoded) that contains implicit or explicit timereferences, as illustrated in FIG. 11. Each sequence is assumed to beregularly repeating at some fixed interval TP in the case of BTS P andTQ in the case of BTS Q and to have a duration that may be equal to orless than the fixed repetition interval. By regular repetition, it ismeant that an identifiable information structure is regularly repeated(e.g., a GSM frame) but not that all, or even any, of the informationcontained in the repeated structure is repeated (although, of course, itmay be). A particular wireless terminal 102, at some location L_(i), isassumed to measure the OTDi between the arrival at the wireless terminal102 of some identified transmission marker M_(Pi) in the transmissionfrom BTS P and another transmission marker M_(Qi) in the transmissionfrom BTS Q. M_(Pi) and M_(Qi) could, for example, be the beginning orend of the repeated transmission sequence or some identifiable point inthe middle. To ensure accuracy, the transmissions should maintain highprecision and stability though transmission drift can be allowed. Thereis no need for synchronized transmissions.

The RTDi between the transmission markers measured by the wirelessterminal 102 is defined as the difference between their absolutetransmission times from each BTS rather than the difference betweentheir arrival times at the wireless terminal 102 (which gives OTDi). TheRTDi is equal to the OTDi if the wireless terminal 102 is equidistantbetween both BTSs. Otherwise, the RTDi and OTDi are related to thedistances between the wireless terminal 102 and BTS P and BTS Q asfollows:

Let T(M_(Ri))=Absolute transmission time of marker M_(Ri) at BTS R; and

-   -   A(M_(Ri))=Absolute arrival time of marker M_(Ri) at Location        L_(i),

where: R=P or Q.

Then: OTDi=A(M _(Pi))−A(M _(Qi))   (6-1)

RTDi=T(M _(Pi))−T(M _(Qi))   (6-2)

$\begin{matrix}{\mspace{20mu} {{{{OTD}\; i} - {{RTD}\; i}} = {{GTD}\; i}}} & \left( {6\text{-}3} \right) \\{\mspace{20mu} {= {\left( {{A\left( M_{Pi} \right)} - {A\left( M_{Qi} \right)}} \right) - \left( {{T\left( M_{Pi} \right)} - {T\left( M_{Qi} \right)}} \right)}}} & \; \\{\mspace{20mu} {= {\left( {{A\left( M_{Pi} \right)} - {T\left( M_{Pi} \right)}} \right) - \left( {{A\left( M_{Qi} \right)} - {T\left( M_{Qi} \right)}} \right)}}} & \; \\{\mspace{20mu} {= \frac{D_{Pi} - D_{Qi}}{c}}} & \; \\{\mspace{20mu} {= \frac{\sqrt{\left( {y_{i} - y_{P}} \right)^{2} + \left( {x_{i} - x_{P}} \right)^{2}} - \sqrt{\left( {y_{i} - y_{Q}} \right)^{2} + \left( {x_{i} - x_{Q}} \right)^{2}}}{c}}} & \left( {6\text{-}4} \right)\end{matrix}$

where: GTDi=Geometric Time Difference at the location L_(i);

-   -   D_(Ri)=distance between L_(i) and BTS R;    -   (x_(R), y_(R)) are horizontal x, y coordinates of BTS R        location; and    -   (x_(i), y_(i)) are horizontal x, y coordinates of location        L_(i).

If the horizontal coordinates for the location L_(i) of the wirelessterminal 102 are obtained independently using, for example, GPS or A-GPSlocation, then there are five unknown variables in Equation (6-4)—the xand y coordinates for each BTS and the RTD between the markers M_(Pi)and M_(Qi). Measurements of the OTDs for the same markers at fivedifferent locations L_(i) (i=1, 2, 3, 4, 5) would then generally besufficient to solve for these five variables.

Measurement of exactly the same markers by the same wireless terminal102 would not be possible since the terminal would necessarily makemeasurements at different times. Because of this, the values for theRTDi (i=1, 2, 3 . . . ) may not all be the same. However, if thedifferences between the RTDs can be obtained then each RTD can beexpressed in terms of one common unknown RTD allowing Equation (6-4) tobe solved for this RTD value and for the coordinates of the two BTSs if5 separate OTD measurements are made by wireless terminal 102 at 5separate locations.

For different measurements made by the wireless terminal 102 atdifferent locations L_(i) (where i=1, 2, 3 . . . ), the RTDs applicableto the measurements made at any two locations L_(i) and L_(k) (wherej≠k) are related as follows:

$\begin{matrix}\begin{matrix}{{{RTDj} - {RTDk}} = {\left\lbrack {{T\left( M_{Pj} \right)} - {T\left( M_{Qj} \right)}} \right\rbrack - \left\lbrack {{T\left( M_{Pk} \right)} - {T\left( M_{Qk} \right)}} \right\rbrack}} \\{= {\left\lbrack {{T\left( M_{Pj} \right)} - {T\left( M_{Pk} \right)}} \right\rbrack - \left\lbrack {{T\left( M_{Qj} \right)} - {T\left( M_{Qk} \right)}} \right\rbrack}}\end{matrix} & \left( {6\text{-}5} \right)\end{matrix}$

Equation (6-5) enables the RTD difference to be obtained if the timeinterval between two identifiable transmission markers from the same BTSis known. This will be possible if the two transmission markers occur inthe same transmission sequence and their relative times of occurrence inthe transmission sequence are known. This will also be possible when thetwo transmission markers occur in different transmission sequences ifthe interval of time from the beginning of each transmission sequence tothe respective transmission marker and the time interval between thebeginnings of the two transmission sequences are all known. The timeinterval between the beginnings of the two transmission sequences can beknown when the time interval between consecutive transmission sequencesis known and fixed and when each transmission sequence carries anexplicit or implicit sequence number, thereby allowing the number oftransmission sequences from that containing the first marker to thatcontaining the second marker to be obtained. If sequences are notnumbered, then there is an ambiguity in the value of the RTD differencein Equation (6-5) that is of the form (n*TP+m*TQ) where n and m arepositive or negative integers, corresponding to the unknown number oftransmission sequences between the markers from each BTS and TP and TQare the assumed fixed intervals between the beginnings of consecutivetransmission sequences from BTS P and BTS Q, respectively. However, ifthe repetition interval for the transmission sequences is the same foreach BTS (i.e., TP=TQ) and is large compared to the maximum absolutevalue of the right hand side of Equation (6-4) (i.e., BTS propagationdelay to any terminal is much less than the repetition interval), thenthe ambiguity in the RTD difference disappears since at most one valuefor the uncertainty (n+m)*TP will provide a solution to Equation (6-4).

If transmission from the BTSs P and Q is not stable but drifts overtime, then the absolute transmission time of any signal S_(R)(t) fromeither BTS R with an intended transmission time of t may drift asfollows:

T(S _(R)(t))=t+a _(R1) t+a _(R2) t ² +a _(R3) t ³+ . . .   (6-6)

where R=P or Q; and

a_(Ri)=fixed small coefficient (where i≧1).

For two transmission markers, one from each BTS, with intendedtransmission times of (t+Δt_(P)) and (t+Δt_(Q)) that are each very closeto t, the RTD as obtained for these transmission markers will beapproximately given by:

$\begin{matrix}{\mspace{20mu} {{{RTD}(t)} = {{T\left( {S_{P}\left( {t + {\Delta \; t_{P}}} \right)} \right)} - {T\left( {S_{Q}\left( {t + {\Delta \; t_{Q}}} \right)} \right)}}}} & \; \\{= {\left\lbrack {\left( {t + {\Delta \; t_{P}}} \right) + {a_{P\; 1}\left( {t + {\Delta \; t_{P}}} \right)} + {a_{P\; 2}\left( {t + {\Delta \; t_{P}}} \right)}^{2} + {a_{P\; 3}\left( {t + {\Delta \; t_{P}}} \right)}^{3} + \ldots} \right\rbrack -}} & \; \\\left\lbrack {\left( {t + {\Delta \; t_{Q}}} \right) + {a_{Q\; 1}\left( {t + {\Delta \; t_{Q}}} \right)} + {a_{Q\; 2}\left( {t + {\Delta \; t_{Q}}} \right)}^{2} + {a_{Q\; 3}\left( {t + {\Delta \; t_{Q}}} \right)}^{3} + \ldots} \right\rbrack & \; \\{\approx {\left( {{\Delta \; t_{P}} - {\Delta \; t_{Q}}} \right) + {\left( {a_{P\; 1} - a_{Q\; 1}} \right)t} + {\left( {a_{P\; 2} - a_{Q\; 2}} \right)t^{2}} + {\left( {a_{P\; 3} - a_{Q\; 3}} \right)t^{3}} + \ldots}} & \left( {6\text{-}7} \right) \\{\mspace{20mu} {\approx {{RTD} + {b_{1}t} + {b_{2}t^{2}} + {b_{3}t^{3}} + \ldots}}} & \left( {6\text{-}8} \right)\end{matrix}$where: RTD=(Δt _(P) −Δt _(Q)); and

b _(i)=(a _(Pi) −a _(Qi)) where i≧1.

Equation (6-7) is obtained by ignoring small terms of order(a_(Ri)Δt_(R)) (where R=P or Q, and i≧1). The resulting RTD in Equation(6-8) contains the true RTD that would apply without any drift plus anumber of error terms corresponding to a linear, quadratic, cubic etc.drift in time.

For linear only drift of the RTD, which results when transmission timingonly has a linear drift which is well known as the most common type ofdrift, the coefficients b_(i) would all be zero for i>1. For quadraticonly drift, b_(i) would all be zero for i>2 and so on. Typically, thehigher coefficients will be zero or almost zero, which means that thenumber of variables to solve for in Equation (6-4), when the RTD inEquation (6-4) comes from Equation (6-8), is increased by the smallnumber of unknown non-zero coefficients in Equation (6-8). By adding thesame number of additional OTD measurements from the terminals at morelocations, the coordinates of the BTSs will still be obtainable.

Once BTS locations have been obtained, Equation (6-4) may be used toobtain the RTD between a pair of BTSs using specific transmissionmarkers that are repeated at the same fixed interval from each BTS. Inthe case of GSM and WCDMA, the use of regularly repeating transmissionframes of a fixed known duration enables such markers to be chosen. Forexample, the markers could be the start of each consecutive GSMtransmission frame. Using such markers, RTDs applicable at differenttimes at different locations can be the same since the terms in Equation(6-5) would be zero. The RTD for any pair of BTSs then defines the timeperiod for which transmission from one BTS leads or lags behind thatfrom another BTS. If the location coordinates of the wireless terminal102 are then obtained and at almost the same time the terminal measuresthe OTD between the pair of BTSs, the RTD is obtainable from Equation(6-4).

The GTD defined in Equation (6-3) for any pair of BTSs depends on thelocation L_(i) and the locations of the BTSs as shown in Equation (6-4),and not on the particular OTD or RTD values. Equation (6-3) shows thatthe GTD for any location can be obtained by measuring the OTD at thatlocation if the RTD is already known (e.g., was obtained previously atsome other location and at some previous time). Equation (6-4) alsoshows that GTDs may be obtained from the location of the terminal andthe locations of the BTSs.

7. Aspects of a Method for Locating a First Network

FIG. 12 is a flow chart illustrating an example of a method for locatinga first network. In step 1202, the difference in transmission time of amarker between two transmitters in a second network is obtained. In oneaspect of this method, this may be achieved by obtaining the location ofeach of the two transmitters and using the locations and the differencesin arrival time of the marker to obtain the difference in transmissiontime of the marker. The location of each of the two transmitters may beobtained using the difference in arrival time of the marker between thetwo transmitters and the location of where the markers are received.Alternatively, the location of one (or both) of the transmitters may beobtained using the arrival time of the marker from said one of thetransmitters and the location of where the marker from said one of thetransmitters is received, or using the signal propagation time of themarker from said one of the transmitters at two different locations. Inan alternative aspect of this method, the difference in transmissiontime of the marker between the two transmitters may be provided by thesecond network. In another alternative aspect of this method, thedifference in transmission time of the marker between the twotransmitters may be obtained by using the signal propagation delay ofthe marker and the time of arrival of the marker for each of the twotransmitters.

In step 1204, the difference in arrival time of the marker between thetwo transmitters is measured.

In step 1206, the first network is located based on the difference intransmission time and the difference in arrival time of the marker. Thefirst network may be located by comparing the difference in transmissiontime and the difference in arrival time of the marker to a fingerprintthe first network. The fingerprint is created by detecting the firstnetwork and recording the difference in transmission time and thedifference in arrival time of the marker when the first network isdetected. Once the first network is located, it may be accessed.

8. Aspects of a Wireless Terminal

FIG. 13 is a functional block diagram illustrating an example of awireless terminal configured to operate in a first network. The wirelessterminal 102 includes a module 1302 for obtaining the difference intransmission time of a marker between two transmitters in a secondnetwork. In one aspect of a wireless terminal 102, this may be achievedby obtaining the location of each of the two transmitters and using thelocations and the differences in arrival time of the marker to obtainthe difference in transmission time of the marker. The location of eachof the two transmitters may be obtained using the difference in arrivaltime of the marker between the two transmitters and the location ofwhere the markers are received. Alternatively, the location of one (orboth) of the transmitters may be obtained using the arrival time of themarker from said one of the transmitters and the location of where themarker from said one of the transmitters is received, or using thesignal propagation time of the marker from said one of the transmittersat two different locations. In an alternative aspect of a wirelessterminal 102, the difference in transmission time of the marker betweenthe two transmitters may be provided by the second network. In anotheralternative aspect of a wireless terminal, the difference intransmission time of the marker between the two transmitters may beobtained by using the signal propagation delay of the marker and thetime of arrival of the marker for each of the two transmitters.

The wireless terminal 102 also includes a module 1304 for measuring thedifference in arrival time of the marker between the two transmitters.

The wireless terminal 102 further includes a module 1306 for locatingthe first network based on the difference in transmission time and thedifference in arrival time of the marker. The first network may belocated by comparing the difference in transmission time and thedifference in arrival time of the marker to a fingerprint of the firstnetwork. The fingerprint of the first network may be created bydetecting the first network and recording the difference in transmissiontime and the difference in arrival time of the marker when the firstnetwork is detected. Once the first network is detected, it may beaccessed by the wireless terminal 102.

The previous description is provided to enable any person skilled in theart to practice the various embodiments described herein. Variousmodifications to these embodiments will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other embodiments. Thus, the claims are not intended to belimited to the embodiments shown herein, but is to be accorded the fullscope consistent with the language claims, wherein reference to anelement in the singular is not intended to mean “one and only one”unless specifically so stated, but rather “one or more.” All structuraland functional equivalents to the elements of the various embodimentsdescribed throughout this disclosure that are known or later come to beknown to those of ordinary skill in the art are expressly incorporatedherein by reference and are intended to be encompassed by the claims.Moreover, nothing disclosed herein is intended to be dedicated to thepublic regardless of whether such disclosure is explicitly recited inthe claims. No claim element is to be construed under the provisions of35 U.S.C. §112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for” or, in the case of a method claim, theelement is recited using the phrase “step for.”

1. A method for a mobile device to locate a wireless local area network(WLAN), the method comprising: recording geometric time difference (GTD)statistics for the WLAN comprising: recording a first minimum GTDbetween a first pair of transmitters; and recording a first maximum GTDbetween the first pair of transmitters; computing a first current GTDbased on current measurements from the first pair of transmitters; andchecking for coverage within the WLAN based on the first current GTDbeing between the first minimum GTD and the first maximum GTD.
 2. Themethod of claim 1, wherein recording the GTD statistics furthercomprises: recording a second minimum GTD between a second pair oftransmitters; and recording a second maximum GTD between the second pairof transmitters; wherein the method further comprises computing a secondcurrent GTD based on current measurements from the second pair oftransmitters; and wherein checking for coverage within the WLAN isfurther based on the second current GTD being between the second minimumGTD and the second maximum GTD.
 3. The method of claim 2, wherein: thefirst pair of transmitters comprise a first base station and a secondbase station; and the second pair of transmitters comprise a third basestation and a fourth base station.
 4. The method of claim 1, whereinrecording the GTD statistics further comprises receiving the firstminimum GTD and the first maximum GTD from a network.
 5. The method ofclaim 1, wherein recording the GTD statistics further comprises:computing the first minimum GTD based on measurements from the firstpair of transmitters while the mobile device is at a first locationwithin coverage of the WLAN; and computing the first maximum GTD basedon measurements from the first pair of transmitters while the mobiledevice is at a second location within coverage of the WLAN.
 6. Themethod of claim 1, wherein the GTD statistics comprises values computedby at least one other mobile device.
 7. The method of claim 1, whereincomputing the first current GTD comprises determining a location foreach of the first pair of transmitters.
 8. The method of claim 7,wherein determining the location for each of the first pair oftransmitters comprises receiving the location for each of the first pairof transmitters from a network.
 9. The method of claim 1, whereincomputing the first current GTD comprises determining a differencebetween an observed time difference (OTD) and a real time difference(RTD) for the current measurements.
 10. The method of claim 9, whereindetermining the RTD comprises receiving the RTD for the first pair oftransmitters from a network.
 11. The method of claim 1, wherein thefirst pair of transmitters belongs to a synchronous network, and whereinthe first current GTD is equal to a first current observed timedifference (OTD) for the current measurements.
 12. A mobile device forlocating a wireless local area network (WLAN), the mobile devicecomprising a processor and memory wherein the memory includes softwareinstructions to: record geometric time difference (GTD) statistics forthe WLAN comprising software instructions to: record a first minimum GTDbetween a first pair of transmitters; and record a first maximum GTDbetween the first pair of transmitters; compute a first current GTDbased on current measurements from the first pair of transmitters; andcheck for coverage within the WLAN based on the first current GTD beingbetween the first minimum GTD and the first maximum GTD.
 13. The mobiledevice of claim 12, wherein the software instructions to record the GTDstatistics further comprises software instructions to: record a secondminimum GTD between a second pair of transmitters; and record a secondmaximum GTD between the second pair of transmitters; wherein thesoftware instructions further comprise software instructions to computea second current GTD based on current measurements from the second pairof transmitters; and wherein the software instructions to check forcoverage within the WLAN is further based on the second current GTDbeing between the second minimum GTD and the second maximum GTD.
 14. Amobile device for locating a wireless local area network (WLAN), themobile device comprising: means for recording geometric time difference(GTD) statistics for the WLAN comprising: means for recording a firstminimum GTD between a first pair of transmitters; and means forrecording a first maximum GTD between the first pair of transmitters;means for computing a first current GTD based on current measurementsfrom the first pair of transmitters; and means for checking for coveragewithin the WLAN based on the first current GTD being between the firstminimum GTD and the first maximum GTD.
 15. The mobile device of claim14, wherein means for recording the GTD statistics further comprises:means for recording a second minimum GTD between a second pair oftransmitters; and means for recording a second maximum GTD between thesecond pair of transmitters; wherein the mobile device further comprisesmeans for computing a second current GTD based on current measurementsfrom the second pair of transmitters; and wherein the means for checkingfor coverage within the WLAN is further based on the second current GTDbeing between the second minimum GTD and the second maximum GTD.
 16. Acomputer program product, comprising a computer usable medium having acomputer-readable program code embodied therein, said computer-readableprogram code adapted to be executed to implement a method for generatinga report, said method comprising: recording geometric time difference(GTD) statistics for the WLAN comprising: recording a first minimum GTDbetween a first pair of transmitters; and recording a first maximum GTDbetween the first pair of transmitters; computing a first current GTDbased on current measurements from the first pair of transmitters; andchecking for coverage within the WLAN based on the first current GTDbeing between the first minimum GTD and the first maximum GTD.
 17. Anon-volatile computer-readable storage medium including program codestored thereon, comprising program code to: record geometric timedifference (GTD) statistics for the WLAN comprising program code to:record a first minimum GTD between a first pair of transmitters; andrecord a first maximum GTD between the first pair of transmitters;compute a first current GTD based on current measurements from the firstpair of transmitters; and check for coverage within the WLAN based onthe first current GTD being between the first minimum GTD and the firstmaximum GTD.